Gas Turbine Engines are used in modern aircraft and other vehicles for both propulsion and auxiliary power. They are also commonly used for electricity production. The reliable operation of these turbine engines is of critical importance. Typical gas turbine engines may be automatically controlled via an engine controller such as, for example, a DEEC (Digital Electronic Engine Controller). The engine controller receives signals from various sensors within the engine, as well as from various pilot-manipulated controls. In response to these signals, the engine controller regulates the operation of the gas turbine engine.
One issue in maintaining reliability in a turbine engine is avoiding lean blowout (LBO), a condition sometimes also referred to as flame out. In general, lean blowout occurs when the fuel flow falls below the level needed to maintain combustion. When a lean blowout occurs the combustion in the turbine engine ceases until it is restarted using the ignition system.
When used for vehicle propulsion the turbine engine must be able to operate over a wide range of speeds and it must be able to change engine speeds at a relatively high rate. For example, the turbine engine must be able to decelerate quickly when needed. This requires that the fuel system be able to reduce fuel flow sufficiently to slow the turbine engine at the needed rate. However, as described above, a low fuel flow can result in a lean blowout, especially when the low fuel flow occurs in a relatively cold engine. Such a lean blowout in a turbine engine is highly undesirable for reliability and safety reasons.
To prevent lean blowout, many turbine engines are designed to follow a lean blowout schedule that defines a minimum fuel flow delivered to the turbine engine based on operating conditions. During operation of the turbine engine the commanded fuel flow is maintained above a minimum value, called the lean blowout schedule. The lean blowout schedule is designed to ensure that lean blowout out does not occur in the engine, while still allowing for sufficient control of the turbine engine for low output and/or deceleration.
Unfortunately, previous techniques for setting the lean blowout schedule have had significant limitations. For example, previous techniques have used fixed lean blowout schedules. However, due to engine and control system variations and differing atmospheric conditions, these fixed lean blowout schedules can be higher than is required for most situations yet lean blowout can still occur in other situations. Thus, the use of fixed lean blowout schedules has reduced engine speed control and/or has been unable to completely eliminate the possibility of lean blowout. Hence, there remains a need for a system and method for controlling fuel flow in a turbine engine that provides needed engine control while further reducing the possibility of lean blowout.